Field of the Invention
The present invention relates to a free-machining steel which does not rely on lead as a means of enhancing machinability. More specifically, the invention relates to a free-machining steel having a concentration of tin at the ferrite grain boundaries of the steel which has machinability comparable to, or better than, that of conventional lead-bearing free-machining steels. The present invention also relates to a process for producing such free-machining steels.
Description of the Related Art
Free-machining steels are utilized in the machining of various components by means of fast-cutting machine-tools. Free-machining steels arc characterized by good machinability, that is, (i) by their ability to cause relatively little wear on the cutting tool thereby extending the useful life of the cutting tool and (ii) by high surface quality. Low tool wear permits the use of higher cutting speeds resulting in increased productivity. The extended cutting tool life further reduces production costs by allowing savings in the cost of cutting tools and in the avoidance of the down time associated with changing cutting tools.
Machinability is a complex and not fully understood property. A full understanding of machinability would require taking into account a multitude of factors, including the effect of the steel composition, the elastic strain, plastic flow, and fracture mechanics of the metal workpiece, and the cutting dynamics that occur when steel is machined by cutting tools in such operations as turning, forming, milling, drilling, reaming, boring, shaving, and threading. Due to the complexities of the cutting process and the inherent difficulties in making real time observations at a microscopic level, knowledge of the extent of the range of mechanisms that affect machinability is also incomplete.
Metallurgists have long assumed that improvements in the machinability of free-machining steels could be obtained by modifying the chemical composition of those steels to optimize the size, shape, distribution, and chemical composition of inclusions to enhance brittleness of the chip and to increase lubrication at the tool/chip interface. They have also sought to prevent the formation of abrasive inclusions which could increase tool wear.
Accordingly, it has been common to use free-machining steels in which soft inclusions, such as manganese sulfide, are dispersed. The manganese sulfide inclusions extend cutting tool service life by bringing about effects such as crack propagation, decrease of cutting tool wear through tool face lubrication, and prevention of cutting edge buildup on the cutting tools. In contrast, hard oxide or carbonitride inclusions, such as silicon oxide, aluminum oxide titanium oxide, titanium carbonitride, which have hardnesses higher than that of the cutting tool, act like fine abrasive particles to abrade and damage the cutting tool thereby decreasing its service life. Thus, free-machining steels are generally not subjected to strong deoxidation during steelmaking so as to keep the content of hard inclusions low.
Historically, lead has been added to free-machining steels containing manganese sulfide inclusions to enhance the machinability of those steels. However, the use of lead has serious drawbacks. Lead and lead oxides are hazardous. Caution must be taken during steelmaking and any other processing steps involving high temperatures. Such process steps produce lead and/or lead oxide fumes. Atmosphere control procedures must be incorporated into high temperature processing of lead-bearing steels. Disposal of the machining chips from lead-bearing free-machining steels is also problematic due to the lead content of the chips. Another serious disadvantage is that lead is not uniformly distributed throughout conventional steel products. This is because lead is not soluble in the steel and, due to its high density, it settles out during the teeming and solidification processes, resulting in segregation or non-uniform distribution within the steel.
Lead's ability to enhance machinability has been attributed to effects that flow from a combination of lead's low melting temperature and its propensity to surround manganese sulfide inclusions as a soft phase. Thus, previous efforts to replace lead in free-machining steels have focused on replicating this combination of characteristics. Consequently, free-machining steels were developed in which a soft phase, such as a low melting metal like bismuth or a plastic oxide, such as a complex oxide containing calcium, took the place of lead in surrounding the manganese sulfide inclusions.